The use of a phase-amplitude array antenna is particularly suited for use in radar systems which are mechanically scanned about one axis and electrically scanned about a second axis. Because of its high degree of versatility, such a radar can simultaneously provide angular elevation and azimuth target position measurements.
The array antenna includes a plurality of vertically and horizontally arranged radiating elements suitably spaced from one another. The relative amplitude and phase of illumination for each element is controlled to obtain a desired combined radiation pattern from the antenna as a whole. The radiating element may be a dipole, waveguide horn, or any other type of antenna. For a more detailed treatment of array antennas, see T. C. Cheston & J. Frank, Chapter 11, Radar Handbook (M. I. Skolnik, Editor; McGraw Hill, 1970).
It is known that phase-amplitude array antenna configurations can uniquely define target position including range, angular elevation and azimuth with respect to a boresight axis. The required information is returned from a target in the pair of signals from two portions of the overall antenna, which for example may be the upper and lower halves of a planar array. These signals constitute a pair of vectors which may be out of phase and of different amplitude, if the target is not on the boresight axis.
It is conventional to label the return vectors A and B, and to consider two reference directions: that of the composite vector A+B and its quadrature, 90 degrees removed therefrom. The vector difference A−B defines two quantities: azimuth and elevation error, respectively. Elevation error is the difference component of A−B in phase with A+B; and azimuth error is the difference component of A−B in quadrature or 90 degrees removed from A+B.
Return vectors A and B are established by separately receiving signals from the selected target from separate regions of the array antenna. The direction of the antenna beam from each section is offset from the boresight axis but the respective beams overlap, permitting a separate return in phase and amplitude from each composite antenna pattern. From these measured phase and amplitude differences, angle and azimuth information can be derived, which can be interpreted to provide an indication of target position.
As will be seen, two sets of phase shifters and a complex antenna and feed structure are required in the construction of a conventional phased array, phase-amplitude monopulse radar system. This is because a typical array antenna employs a flat planar arrangement of radiators which often are end-fed from the feed structure. Normally, the array is rotated in an azimuth about a vertical axis, and elevation scan is electronic.
This physical rotation makes it difficult to effectively derive an azimuthal monopulse. Elevation monopulse is much more conveniently established through an end-fed array and monopulse feed network providing both a sum channel (for weighted summation of signals for all the rows), and an elevation difference signal channel (for weighted summation of difference signals between corresponding upper and lower rows).
Conventionally, the dividing network on each row can be designed to provide an azimuth difference signal as well as the sum signal. Additional phase shifters and a summing network can be provided to establish an azimuth difference signal. However, it is difficult to obtain a useable azimuth difference signal while maintaining effective frequency scan in azimuth associated with the end row feed rows, while concurrently maintaining low sidelobe characteristics.
Accordingly, it is an object of the invention herein to eliminate the need for the complex feed structures required by conventional rotating 2-axis monopulse systems.
It is additionally an object of the invention to eliminate the need for two sets of phase shifters as employed in the current technology.
It is a further object of the invention to establish an array antenna arrangement with very low sidelobes.
It is another object of the invention to obtain the azimuth difference signal while maintaining the low sidelobe and frequency scan characteristics of an end-feed array.
It is even another object of the invention to provide an improved phase-amplitude monopulse configuration in which the array is divided into upper and lower faces, which are warped left and right in azimuth.